A recent publication in The Lancet: Diabetes & Endocrinology is prompting the medical community to rethink how obesity is screened and diagnosed. Endorsed by over 75 organizations worldwide, including scientific societies and patient advocacy groups, this consensus calls for a major overhaul of the language, classification, and diagnostic criteria used to address excess adiposity and its impact on the body.

Interestingly, the commission not only includes international experts from diverse medical specialties and regions but also individuals with lived experience of obesity. Together, they aim to define ‘clinical obesity’ and establish objective criteria for its diagnosis. Let’s dive into their final recommendations.

What is Obesity?

The definition of obesity has historically been steeped in judgment and stigma, often shaping perceptions and attitudes toward those living with it. Obesity has frequently been framed as a “state of being” rather than a medical condition someone “has,” perpetuating blame and misconceptions about its causes.

For instance, the Oxford English Dictionary defines obesity as “the condition of being extremely fat or overweight; stoutness, corpulence.” This wording carries undertones that reinforce negative stereotypes and fails to address the medical complexities of obesity.

Webster’s Dictionary offers a slightly more clinical approach, defining obesity as “a condition characterized by the excessive accumulation and storage of fat in the body.” While less stigmatizing, this definition still lacks nuance.

The World Health Organization (WHO) provides a more health-centered definition: “abnormal or excessive fat accumulation that presents a risk to health,” which emphasizes the medical and health-related aspects of obesity.

While these definitions show progress in understanding obesity, they still fall short of fully addressing the stigma or providing a science-based perspective on the condition.

How is Obesity Diagnosed?

The effort to establish quantifiable characteristics of the “normal man” dates back to the early 1800s, when Belgian statistician, mathematician, and astronomer Adolphe Quetelet developed the Quetelet Index—weight divided by height squared—through his passion for statistical analysis and bell-shaped curves.

In the 1950s, Louis Dublin, a statistician and vice president of the Metropolitan Life Insurance Company, expanded on this concept by creating tables of “normal weights” for clients. These tables emerged after the company observed an increasing number of claims from obese policyholders. Although Dublin’s tables did not account for age or gender, they categorized clients into three frame sizes: “small,” “medium,” and “large.”

In the 1970s, physiologist Ancel Keys built upon Quetelet’s work and officially coined the modern term “Body Mass Index” (BMI).

Fast forward to 2025, BMI remains the predominant diagnostic tool for obesity. According to WHO guidelines, overweight is classified as a BMI of 25–29.9 kg/m², while obesity is defined as a BMI of 30 kg/m² or higher.

The Limitations of BMI

It should go without saying that BMI has clear limitations as a diagnostic tool, not only for assessing body adiposity but also for evaluating overall wellness and long-term health outcomes.

BMI does not account for body composition, such as the proportion of body fat versus lean skeletal muscle mass. Additionally, it fails to recognize variations in fat distribution (e.g., apple vs. pear body shapes or visceral vs. subcutaneous fat), as well as critical factors like age, sex, gender, race, or ethnicity—all of which can significantly influence health outcomes.

Ilona Maher, a U.S. Women’s Rugby player, recently highlighted the challenges of using BMI to assess the athleticism of U.S. Olympic athletes. Standing 5 feet 10 inches tall and weighing 200 pounds, Ilona’s BMI calculates to just under 30, technically classifying her as obese. However, she shared data from her recent body composition analysis, revealing 170 pounds of lean muscle mass, which indicates a very low body fat percentage and highlights the inadequacy of BMI in accurately assessing athletic individuals.

Consequences of Misdiagnosing Obesity

The consequences of misdiagnosing obesity can be significant and far-reaching, including delayed or inappropriate care, psychological and emotional impacts, missed opportunities for early intervention, and misguided public health policies.

Redefining Obesity

Consequently, there is an urgent need to redefine obesity as a distinct clinical condition, where health risks associated with excess adiposity are objectively documented through specific signs and symptoms.

In response, the authors of the new consensus statement define obesity as “a condition characterized by excess adiposity, with or without abnormal distribution or function of adipose tissue, and with causes that are multifactorial and still incompletely understood.” They also sought to establish clear clinical and biological criteria for diagnosing “clinical obesity,” framing it similarly to other chronic diseases to reflect its status as an ongoing illness.

Therefore, clinical obesity is defined as “a chronic, systemic illness characterized by alterations in the function of tissues, organs, the entire individual, or a combination thereof, due to excess adiposity.”

Preclinical vs. Clinical Obesity

The consensus suggests that obesity exists on a biological continuum. For practical purposes, it can be categorized into preclinical obesity (without clinical manifestations) and clinical obesity (characterized by signs, symptoms, or impaired daily functioning).

Preclinical Obesity

Preclinical obesity is defined as “a state of excess adiposity with preserved function of tissues and organs, accompanied by an increased risk of developing clinical obesity and associated diseases, such as type 2 diabetes, cardiovascular disease, and certain types of cancer.”

It’s recommended that people with preclinical obesity receive evidence-based health counseling and regular monitoring of their health status. Preclinical obesity may also require interventions to reduce the risk of progressing to clinical obesity and developing other obesity-related diseases.

Preclinical obesity typically does not require treatment with medications or surgery.

Clinical Obesity

To diagnosis clinical obesity, the following is required:

Clinical confirmation of obesity status by anthropometric criteria or by direct body fat measurement.

Plus, one or both of the following criteria:

Evidence of impaired organ or tissue function caused by obesity, such as signs, symptoms, or diagnostic tests indicating abnormalities.
Substantial, age-adjusted limitations in activities of daily living caused by obesity, including tasks like walking, bathing, dressing, toileting, continence, or eating.
Or both.

Clinical obesity can lead to severe end-organ damage, resulting in life-altering and potentially life-threatening complications. Individuals with clinical obesity should receive timely, evidence-based treatment to improve obesity-related clinical manifestations and prevent further progression.

Measuring Adiposity to Determine Obesity

Distinguishing preclinical obesity from clinical obesity requires an accurate assessment of body fat. For rugby player Ilona Maher, BMI alone proved to be an inaccurate measure of adiposity.

The clinical assessment of obesity requires confirmation of excess or abnormal adiposity using one of the following methods:

Direct body fat measurement, such as dual-energy X-ray absorptiometry (DEXA) or bioimpedance analysis.
At least one anthropometric criterion (e.g., waist circumference, waist-to-hip ratio, or waist-to-height ratio) in addition to BMI.
At least two anthropometric criteria (e.g., waist circumference, waist-to-hip ratio, or waist-to-height ratio) regardless of BMI.

Confirmation of obesity status defines a physical phenotype but does not constitute a disease diagnosis in itself. Therefore, individuals should be further evaluated for clinical obesity. In cases of significantly high BMI levels (i.e., >40 kg/m²), excess adiposity can be reasonably assumed.

Does BMI Still Have a Role?

Yes—but with limitations. BMI serves as a useful screening tool for identifying individuals who may have excess or abnormal adiposity. However, confirming obesity status requires verification through direct body fat measurement or at least one additional anthropometric criterion.

BMI is also a valuable tool for assessing health risks at a population level in epidemiological studies; however, it is not suitable for determining individual health.

Key Takeaways

While redefining diagnostic criteria and refining obesity classifications based on presentation may seem like ‘splitting hairs,’ the ultimate goal is to provide more comprehensive and respectful patient care.

The consensus emphasizes that all individuals with excess adiposity should undergo evaluation for clinical obesity, including a thorough medical history, physical examination, standard lab tests, and additional diagnostic assessments as needed. Those with preclinical obesity should receive evidence-based counseling and care to reduce their risk of developing clinical obesity and related conditions.

Both clinical and preclinical obesity require regular monitoring and screening for type 2 diabetes and other metabolic disorders commonly associated with obesity.

As healthcare professionals, I encourage us to embrace these refined diagnostic criteria to deliver more inclusive and effective care. By working together, we can take meaningful steps to reduce risks and promote better health for all.

References

Rubino F, Cummings DE, Eckel RH, et al. Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology. Published online January 2025:S2213858724003164. doi:10.1016/S2213-8587(24)00316-4
Obesity. Accessed January 25, 2025. https://www.who.int/health-topics/obesity
Pray R, Riskin S. The history and faults of the body mass index and where to look next: a literature review. Cureus. 15(11):e48230. doi:10.7759/cureus.48230
Westbury S, Oyebode O, van Rens T, Barber TM. Obesity Stigma: Causes, Consequences, and Potential Solutions. Curr Obes Rep. 2023;12(1):10-23. doi:10.1007/s13679-023-00495-3